There is no clear system in DF, thats the problem.
There is clothing. A shirt. But you can also make a shirt from netherbark leather, which is steel grade and temperature immune.
There is armor. Lets say a chainmail. But you can make that from copper.
In that case, the shirt (netherbark) would be way better than the armor (copper). But if you make the chainmail out of steel, it would be better, because it covers more, but it would be worse in incidents that include fire.
In theory it goes like this in MDF:
Shirts (clothing) => padded shirt (armor, leather, can be worn under armor) => chainmail (armor, metal, can be worn over armor) => breastplate (metal) => platemail (metal).
Best would be wearing padded armor under, platemail as armor, and chainmail over it.